Method of producing distributing reflectors, and reflectors provided by the method

ABSTRACT

A method of producing a distributed reflector that includes a grating, wherein regions ( 21 ) are provided in the grating material transversely to the longitudinal axis of the grating, wherein the refractive index in said regions is lower or higher than the refractive index in surrounding parts of the grating, and wherein the distance between mutually adjacent regions ( 22 ) is varied. The invention is characterised by giving the regions ( 21 ) mutually the same width, and determining the positions of the various regions along the longitudinal axis (X) of the grating in relation to the wavelengths to be reflected.

The present invention relates to a method of producing distributingreflectors, primarily for tuneable lasers, and also to reflectorsproduced in accordance with the method.

Tuneable semiconductor lasers include several sections through whichcurrent is injected, these sections typically being three or four innumber. The wavelength, power and mode purity of the lasers can becontrolled by regulating the current injected into the various sections.Mode purity implies that the laser is at an operation point, i.e. at adistance from a combination of the drive currents where so-called modejumps take place and where lasering is stable and side mode suppressionis high.

Special requirements are placed on different applications with respectto controlling wavelength. In the case of telecommunicationsapplications, it is necessary that the laser is able to retain itswavelength to a very high degree of accuracy and over long periods oftime after having set the drive currents and the temperature. A typicalaccuracy in this respect is 0.1 nanometer and a typical time period is20 years.

The distributed Bragg reflector (DBR) has played a very significant partin the development of modern semiconductor lasers. The distributed Braggreflector enables the selection of a narrow wavelength range, therewithenabling a single longitudinal oscillation mode to dominate strongly,which means, in turn, that the spectral width of the laser light will bevery small. A multiple wavelength reflector can be created, by modifyinga DBR. Any one of these wavelengths can be selected with the aid of aspectral selection mechanism, therewith producing the basis of a highlytuneable laser. There are many reasons for using tuneable lasers inwavelength-multiplexed optical networks, for instance. One use is as abackup laser for a number of other lasers of fixed wavelength. Thetelecommunications market places high requirements on the componentsused. A tuneable laser must therefore provide a comparable alternativeto a laser of fixed wavelength. The reflector is a very important partof the laser, where enhanced reflectance is highly significant tocomponent performance.

It is known to construct a grating reflector with multiple reflectionpeaks. The Fourier relationship between, the perturbation of thewaveguide and the reflection spectrum has been used as a tool in thisrespect. A sample distributed Bragg reflector can therewith beidentified as a possible way of achieving simultaneous reflections of anumber of narrow peaks and different wavelengths.

The use of frequency-modulated uniform gratings to create a suitablereflector design has also been proposed. A reflector having multiplewavelengths is obtained by producing a series of two or more identicalfrequency modulated gratings. A reflector of this kind is called asuperstructure grating (SSG).

Another method referred to as the binary superimposed grating (BSG) hasrecently been presented. This method lacks the SSG superstructure.

In the BSG method, the magnitude of a design function is used todetermine whether or not the waveguide shall include a material having alow index or a material having a high index.

The contribution of this method to known technology lies in the binaryapproach with a constant s, where s is a distance in which therefractive index has a constantly high or low value, and also in theselection mechanism for a high and a low index.

The method implies the use of a grating that includes a number ofgrooves or lines where the width of the grooves varies between differentgrooves and where all grooves have a width which is a multiple of s, andwhere the distance between mutually adjacent grooves also varies with amultiple of s. Thus, both groove width and groove interspacing vary.

This variation in groove width makes manufacture difficult.

An object of the present invention is to enable a grating of this kindto be produced in a much simpler way.

Accordingly, the present invention relates to a method of producing adistributed reflector which includes a grating and which is a multiplewavelength reflector, and in which method the grating is provided withregions (21) in the grating material that lie transversely to thelongitudinal axis of the grating and in which the refractive index iseither lower or higher than in a surrounding part of the grating andwhere the distance between mutually adjacent regions can be varied,wherein the method is characterised by causing said regions to havemutually the same width, and by determining the positions of thedifferent regions along the longitudinal axis of the grating in relationto the wavelength to be reflected.

The invention also relates to a reflector of the kind that has thecharacteristic features set forth in claim 7.

The invention will now be described in more detail with reference toexemplifying embodiments thereof and also with reference to theaccompanying drawings, in which

FIG. 1 is a partially cut-away perspective view of a DBR

FIG. 2 is a sectional view of a tuneable Grating Coupled SampledReflector (GCSR) laser;

FIG. 3 is a sectional view of a Sampled Grating DBR laser;

FIG. 4 is a diagrammatic illustration of reflectivity as a function ofwavelength;

FIG. 5 is a longitudinal sectioned view of part of a grating; and

FIG. 6 is a diagram in which reflectivity is plotted against wavelengthin respect of a communications laser in which the invention is applied.

FIG. 1 illustrates a DBR laser which includes three sections, namely aBragg reflector 1, a phase section 2 and a gain section 3. Each sectionis controlled by injecting current thereinto through respectiveelectrical conductors 4, 5, 6.

FIG. 2 is a section view of a tuneable Grating Coupled SamplingReflector (GCSR) laser. Such a laser has four sections, namely a Braggreflector 7, a phase section 8, a coupler 9 and a gain section 10. Eachof the sections is controlled by injecting current thereinto.

FIG. 3 is a section view of a Sampled Grating DBR laser, which also hasfour sections referenced 11, 12, 13, 14 respectively. The sections 11and 14 are Bragg reflelctors, whereas section 13 is the phase sectionand section 12 the gain section.

The aforesaid three types of lasers are common. However, other types oflasers exist.

The invention is not restricted to the use of the reflector with anyparticular type of tuneable semiconductor laser, but can be applied withtuneable lasers of kinds other than those described and shown by way ofexample in the accompanying drawings.

As before mentioned, the inventive method concerns the production of adistributed reflector, primarily for a tuneable, laser, which includes agrating which is a multiple wavelength reflector, wherewith the gratingis provided in the grating material with regions that lie transverselyto the longitudinal axis of the grating, wherein the refractive index insaid regions is lower than or higher than the refractive index of thesurrounding material, and wherein the distance between mutually adjacentgrooves can be varied.

According to one preferred embodiment of the invention, said regionsconsist of physical grooves in the grating material.

According to another embodiment, said regions are produced byirradiating the grating material in a known manner, such as to changethe refractive index in the irradiated regions, for instance with theaid of ultraviolet light.

According to one preferred embodiment, the reflector is integrated witha tuneable laser which can be tuned to any wavelength selected from thewavelengths that can be reflected.

The inventive reflector is not, however, restricted for use togetherwith a tuneable laser. For instance, the inventive distributed reflectorcan be used as a component in the implementation of advanced filterfunctions in, for instance, a tuneable channel-drop filter for whichthere is chosen for reflection one wavelength from several incidentwavelengths.

FIG. 4 illustrates schematically the reflectivity R as a function of thewavelength λ of such a reflector. Tuning to a particular wavelength thatis included in said wavelengths is achieved by adjustment of thecurrents injected into the different sections of the laser or byadjustment of the voltage across said different sections.

According to the invention, the grooves or lines are given mutually thesame width and the positions of the different grooves along thelongitudinal axis of the grating are determined in relation to thosewavelengths to be reflected.

The present invention is thus grounded on the possibility of producing awavelength-selective reflector of the present kind simply by varying thewidth of the ribs defining the grooves instead of varying both the widthof the ribs and the width of the grooves.

FIG. 5 is a longitudinal sectioned view of part of an inventive gratingwhere the aforesaid regions are comprised of physical grooves. FIG. 5shows grooves 21 of mutually identical widths. Also shown are ribs 22between the grooves 21. These ribs 22 have mutually different widths,depending on the wavelengths to be reflected. The boundaries of thegrating periods are shown in vertical broken lines 23.

In one highly preferred embodiment of the invention, the positions ofthe grooves 21 along the grating are determined in the following way.

A grating that consists of an optical conductor with a sinusoidaldisturbance in refractive index in the longitudinal direction reflectsmainly light in a narrow spectral area whose centre wavelength isdependent on the disturbance period according to λ=2n Λ, where λ iswavelength, n is the mean refractive index and Λ is the disturbanceperiod (hereinafter called the grating period).

One simple way of constructing a grating that reflects a multiple ofwavelengths, is to superpose analogously a plurality of sinusoidaldisturbances with mutually varying grating periods. Such a grating canbe described very generally by the following function:${f(x)} = {\sum\limits_{j = 1}^{N}\quad {a_{j}{\exp \left\lbrack {\left( {\frac{2\quad \pi \quad x}{\Lambda_{j}} + \Psi_{j}} \right)} \right\rbrack}}}$

where the true index refractive disturbance is obtained by the true partof the function f(x). In the formula, N is the number of wavelengthswhere reflection is desired, x is the longitudinal position in thegrating, a_(j) designates selective true coefficients, Λ_(j) is thegrating period of the wavelength j, and where Ψ_(j) designates selectivephases.

Assume that the grating has a total length L_(g) that is an integermultiple of any selected grating period which, for instance, can be sentto the centre wavelength of the reflector, i.e. L_(g)=Nlines*Λ₀, whereΛ₀ is hereinafter referred to as the fund amental period of the gratingand Nlines is a whole number.

The sinusoidal index disturbance described by the fundamental period ofthe grating along the length of said grating is taken as a startingpoint. A single grating period, i.e. the interval in the X-directiondescribed by [(k−1)*Λ₀, k*Λ₀], where k is an integer belonging to[1,Nlines], accommodates precisely one period of the sinusoidaldisturbance. When the index disturbance is Fourier-expanded over saidinterval, the spectrum is caused to include solely one fundamental tonewith no harmonics. It is now possible to introduce a phase shift φ_(k)on the sinus function over the k:nth interval. This can assume valuesbetween 0 and 2π and corresponds to a physical displacement of theposition of the index disturbance. An attempt is now made to imitate thefunction f(x) with a grating that has the fundamental period and withthe aid of all phase shifts φ_(k). This is achieved by Fourier seriesexpansion of the function f(x) over the k:nth interval [(k−1l)*Λ₀,k*Λ₀]. The phase shift φ_(k) of the fundamental tone in the Fourierseries expansion is then determined. The result is a grating that hasthe fundamental period Λ₀, which is phase shifted piecemeal so as toresemble f(x) and thereby obtain the desired reflection properties.

Finally, the refractive index disturbance is identified in a productionprocess that is more like a square wave that has a higher and lowerrefractive index than a sine wave. In this case, the transition betweenthe two wave configurations is clarified by arranging a distance that isequal to half a fundamental period of lower refractive index at theposition where the sine function has a minimum. This results in afavourable situation in which the groove width in the grating isconstant.

FIG. 6 illustrates by way of example reflectivity as a function ofwavelength when an inventive reflector is used. It is thus possible toproduce a complete channel plane for communications purposes, wherereflectivity is essentially equally as good for all selectivewavelengths.

According to one preferred embodiment, openings are formed in asubstrate-mounted resist with the aid of an electron beam in a knownmanner, said openings having a width that corresponds to the width ofsaid regions or areas 21, whereafter the resist is either etched orirradiated with light.

The invention also relates to a reflector of the aforesaid kind in whichthe grooves or lines have mutually the same width and in which thevarious grooves have different positions along the longitudinal axis ofthe grating corresponding to the wavelengths to be reflected.

It will be obvious that the inventive method enables reflectors to beproduced in a much simpler way than is possible with known techniques,owing to the fact that grooves of solely one width need be produced.

Although the invention has been described above with reference toseveral embodiments, it will be understood that the method ofcalculating the positions of the grooves can be varied.

The present invention is therefore not restricted to the aforedescribedembodiments, since variations can be made within the scope of theaccompanying claims.

What is claimed is:
 1. A method of producing a distributed reflectorthat includes a grating, wherein regions are provided in the gratingmaterial transversely to the longitudinal axis of the grating, whereinthe refractive index in said regions is lower or higher than therefractive index in surrounding parts of the grating, and wherein thedistance between mutually adjacent regions is varied, comprising givingthe regions mutually the same width, and determining the positions ofthe various regions along the longitudinal axis (X) of the grating inrelation to the wavelengths to be reflected.
 2. A method according toclaim 1, comprising applying a design function that is a super positionof spatial harmonics:${f(x)} = {\sum\limits_{j = 1}^{N}\quad {a_{j}{\exp \left\lbrack {\left( {\frac{2\quad \pi \quad X}{\Lambda_{j}} + \Psi_{j}} \right)} \right\rbrack}}}$

where N is the number of wavelengths, x is length, a_(j) designatesselective coefficients, Λ_(j) designates the grating period forwavelength j, and where Ψ designates selective phase shifts, and whereΛ₀ is any selected grating period that preferably corresponds to thecentre wavelength of the reflector; and Fourier series expanding thefunction over the interval [(k-1)*Λ₀, k*Λ₀], where k is a whole numberbelonging to [1,Nlines], where Nlines is the number of grating periodsfor determining the phase of the fundamental tone; and determining phaseshift Ψ_(k) from the fundamental tone, and determining the position ofan associated grating opening, i.e. said regions, from said phaseshifts.
 3. A method according to claim 1, further comprising forming thereflector as an integral part of a tuneable laser which can be tuned toany one wavelength selected from those wavelengths that can be reflectedby the reflector.
 4. A method according to claim 1, further comprisinggiving said region the form of physical grooves.
 5. A method accordingto claim 1, further comprising giving said regions the form of regionsthat have been irradiated so that the refractive indexes in said regionswill be lower or higher than the refractive index of the surroundinggrating material.
 6. A method according to claim 1, further comprisingforming in a substrate-mounted resist with the aid of an electron beamopenings that have a width which corresponds to the width of saidregions, and thereafter etching or irradiating said resist.
 7. Adistributed reflector which includes a grating and which is a multiplewavelength reflector, wherein the grating includes in the gratingmaterial regions that lie transversely to the longitudinal axis of thegrating and which have a refractive index which is lower or higher thanthe refractive index of the surrounding part of the grating and wherethe distance between mutually adjacent regions is varied, wherein theregions have identical widths; and in that the regions have mutuallydifferent positions along the longitudinal axis (X) of the gratingcorresponding to the wavelengths to be reflected.
 8. A distributingreflector according to claim 7, wherein the reflector is an integralpart of a tuneable laser, wherein the laser can be tuned to any one ofthe wavelengths that can be reflected by the reflector.
 9. Adistributing reflector according to claim 7 wherein the regions comprisephysical grooves.
 10. A distributing reflector according to claim 7wherein said regions comprise regions that have been irradiated to causethe refractive index of said regions to be lower or higher than therefractive index of the surrounding grating material.